Ruptured Ovarian Cyst: This condition is highly likely given the sudden onset of severe pain, significant tenderness in the right lower abdomen, and the presence of free fluid behind the bladder on the FAST scan, which suggests internal bleeding. The patient's symptoms of passing out, tachycardia, and mild hypotension further support this diagnosis due to potential hypovolemic shock.

Appendicitis: Although the presence of free fluid might suggest a rupture, appendicitis is a common cause of severe right lower abdominal pain, especially with rebound and guarding. The symptoms and physical exam findings align closely with appendicitis, making it a strong consideration.
Ectopic Pregnancy: Given the patient's age and symptoms, an ectopic pregnancy must be considered, especially if there's a possibility of the patient being pregnant. The severe pain and free fluid could indicate a ruptured ectopic pregnancy, which is a medical emergency.
Ovarian Torsion: This condition involves the twisting of an ovary, which can cause severe pain and might lead to necrosis if not promptly treated. The sudden onset of pain and the location could suggest ovarian torsion, although the presence of free fluid is less typical.

Ruptured Ectopic Pregnancy: As mentioned, this is a medical emergency that can quickly lead to severe hemorrhage and death if not promptly diagnosed and treated. The presence of free fluid and severe pain necessitates ruling out pregnancy and considering this diagnosis.
Perforated Visculitis or Diverticulitis: Although less common in a young patient, any condition that leads to perforation of the bowel can cause severe pain, rebound tenderness, and free fluid. These conditions are less likely but critical to diagnose due to their high morbidity and mortality if not treated promptly.
Hemorrhagic Ovarian Cyst: Similar to a ruptured ovarian cyst but might not always present with as clear signs of rupture. The presence of free fluid and severe pain warrants consideration of any ovarian pathology.

Intussusception: A condition where a part of the intestine telescopes into another, which can cause severe abdominal pain and potentially lead to bowel ischemia. It's rare in adults and even more so in young adults without underlying conditions but could present with similar symptoms.
Mesenteric Infarction: This involves the sudden onset of insufficient blood flow to the intestine, leading to ischemia. It's more common in older patients or those with vascular disease but can occur in anyone and presents with severe abdominal pain out of proportion to physical findings.
Intra-abdominal Testicular Torsion: In cases of undescended testes, torsion can occur within the abdomen and present with severe lower abdominal pain. This is rare and typically considered in the context of known undescended testes or other specific risk factors.